As a gear pump device for rubber extrusion, for example, there have been proposed one described in the following Patent Document 1. This proposed gear pump device has, as shown in FIG. 6, a pair of gears (a, a) engaged with each other, a housing c having a gear-housing bore b for housing a pair of the gears (a, a), and a side plate d inserted in the gear-housing bore b.
In a supporting hole of each side plate d, a ceramic cylindrical bushing e is disposed. Then, a support-shaft portion a1 of each gear (a) is pivotally supported via the bushing e.
This device is configured so that a portion of rubber within a pump chamber b1 defined between the side plates d, d is circulated, toward the pump chamber b1, through a gap between an inner peripheral surface of the bushing e and an outer peripheral surface of the support-shaft portion a1.
In this case, as the rubber flows in the direction of the circulation within the gap, the rubber scorch within the gap can be reduced to some degree. But it is not enough, and maintenance is necessary on a regular basis (for example, every two or three months).Further, depending on the kind of the rubber, the rubber scorch is caused early, and the torque which is increased thereby may possibly lead the drive system to fail.